It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid developers are comprised of thermoplastic resin and dispersant nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 30 .mu.m average size as measured using a Malvern 3600E Particle Sizer described below. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said dispersant nonpolar liquid and the image may subsequently be transferred to a carrier sheet and fused to the carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and preferably adjuvants, e.g., polyhydroxy compounds, aminoalcohols, polybutylene succinimide, an aromatic hydrocarbon, metallic soaps, etc., to the liquid developer comprising a thermoplastic resin having free carboxyl groups, dispersant nonpolar liquid, and preferably a colorant. Such liquid developers provide images of good resolution and charging but it has been found that image quality is deficient. The toned and transferred images have a speckled or mottled appearance after the fusing step. In order to overcome this problem much research effort has been expended to develop new types of mottle prevention additives for electrostatic liquid toners.
It has been found that the above disadvantages can be overcome and improved developers prepared containing a dispersant nonpolar liquid, charge director compound, a thermoplastic resin having free carboxyl groups, optionally a colorant, and a mottle prevention additive of the invention. The improved electrostatic liquid developer when used to develop an electrostatic image results in improved image quality, transfer efficiency and improved solid area coverage independent of the pigment and charge director present.